Where is the option for grease pencil convert to curve 2.78a
This function is not in the properties panel (N) but in the tools panel (T). Open it up and scroll down to the Grease Pencil tab on the left. At the bottom of the Grease Pencil panel you can find Convert to Geometry.... Click it and you can choose whether you want to convert it to a polygon curve, a path or a bezier curve.